RF power amplifiers used for wireless communication transmitters, with spectrally efficient modulation formats, require high linearity to preserve modulation accuracy and to limit spectral regrowth. Typically, a linear amplifier, Class-A type, Class-AB type or Class-B is employed to faithfully reproduce inputs signals and to limit the amplifier output within a strict emissions mask. Linear amplifiers are capable of electrical (DC power in to RF power out or DC-RF) efficiencies of 50% or more when operated at saturation. However, they are generally not operated at high efficiency due to the need to provide high linearity. For constant envelope waveforms, linear amplifiers are often operated below saturation to provide for operation in their linear regime. Time varying envelopes present an additional challenge. The general solution is to amplify the peaks of the waveform near saturation, resulting in the average power of the waveform being amplified at a level well backed-off from saturation. The back-off level, also referred to as output power back-off (OPBO), determines the electrical efficiency of a linear amplifier.
For example, the efficiency of a Class-A type amplifier decreases with output power relative to its peak value (EFF=POUT/PPEAK). The efficiency of Class-B type amplifiers also decreases with output power relative to its peak value (EFF=(POUT/PPEAK)1/2). Class-AB type amplifiers have output power variations intermediate between these values. Thus, there is customarily an inherent tradeoff between linearity and efficiency in amplifier designs.
Modern transmitters for applications such as cellular, personal, and satellite communications employ digital modulation techniques such as quadrature phase-shift keying (QPSK) in combination with code division multiple access (CDMA) communication. Shaping of the data pulses mitigates out-of-band (OOB) emissions from occurring into adjacent channels but produces time-varying envelopes. In addition to amplifying individual waveforms with time varying envelopes, many transmitters (especially in base stations) are being configured to amplify multiple carriers. Multi-carrier signals have a wide distribution of power levels resulting in a large peak-to-average ratio (PAR). Therefore, the operation of the linear amplifiers in these types of signals is very inefficient, since the amplifiers must have their supply voltage sized to handle the large peak voltages even though the signals are much smaller a substantial portion of the time. Additionally, the size and cost of the power amplifier is generally proportional to the required peak output power of the amplifier.
Wideband Code Division Multiple Access (WCDMA), Orthogonal Frequency Division Multiplexing (OFDM), and multi-carrier versions of Global Standard for Mobile Communication (GSM) and Code Division Multiple Access 2000 (CDMA 2000) are wireless standards and applications growing in use. Each requires amplification of a waveform with high PAR levels, above 10 dB in some cases. The sparse amount of spectrum allocated to terrestrial wireless communication requires that transmissions minimize OOB emissions to minimize the interference environment. A linear amplifier used to amplify a waveform with a PAR of 10 dB or more provides only 5–10% DC-RF efficiency. The peak output power for the amplifier is sized by the peak waveform. The cost of the amplifier scales with its—peak power. Several other circuit costs including heat sinks and DC-DC power supplies scale inversely to peak power and dissipated heat (which results from the electrical inefficiency). Related base station costs of AC-DC power supplies, back-up batteries, cooling, and circuit breakers also scale inversely with efficiency as does the electrical operating costs. Clearly, improving DC-RF efficiency is a major cost saver both for manufacture and operation.
One of the techniques used in the design of highly linear amplifiers is known as the feedforward (FF) technique. FF amplifier systems are based on a two-loop analog system design. Typically, the first loop splits off a sample of the analog amplifier input as a reference and compares it to a sample of the amplifier output to produce an error signal. The error primarily consists of the signal produced by non-linear amplification in the power amplifier. The second loop subtracts the error signal from a delayed version of the power amplifier output. FF is effective in reducing OOB emissions and to some repairing errors to the wanted signal. The technique requires careful control of the amplitude and phase of each signal, which impacts the maximum bandwidth over which a level of correction can be applied. A range of techniques exists to control amplitude and phase over temperature variations. If there is any corruption of the reference then the performance of the FF system degrades. Ideally, the FF technique reduces the amplifier distortion level to that of the input. The amplifier input is generally already distorted somewhat from non-linearities or clipping effects earlier in the amplifier signal chain (e.g., DAC, mixers, and/or driver amplifiers.)
Feed forward techniques have several limitations. They cannot be employed in amplification systems that decompose an input signal into multiple components (e.g., polar amplifier, LINC amplifier), since a single input signal is not available as a reference. If the input signal the amplifier is highly distorted, for example due to an element that clips the peak of the signal, then FF will not significantly reduce overall distortion. It is not possible to combine digital pre-distortion with feedforward as the reference signal will contain the amplitude and phase changes as a result of the pre-distortion, and these changes will be contained in the error signal and re-introduced to the signal at the amplifier output.